Method of forming a superconductor film

ABSTRACT

The present invention provides methods forming the superconductor of as-grown film of MgB 2  which is made with magnesium and boron ejected from a magnesium target and a boron target, respectively, each in simultaneously sputtering process. The as-grown film composed of a compound of magnesium and boron is a superconductor without being annealed. The present invention can be applied to fabricate an integrated circuit of superconductor film, because the high temperature annealing process for the as-grown film of MgB 2  is unnecessary.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No.2001-356802, filed Nov. 22, 2001 in Japan, the contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of forming a superconductorfilm composed of a compound of magnesium (Mg) and boron (B). Morespecifically, the ]present invention relates to a method of forming anas-grown film having a high critical temperature superconductor withoutbeing annealed.

2. Description of the Related Art

A MgB₂ compound features one of the highest transition temperatures ofany superconductor. Additionally, a MgB₂ film can be integrated on asubstrate to form an integrated circuit. Thus, the MgB₂ compound isexpected to apply to electronics devices such as thin film devices.

It is reported that the coherent length ξ(0) of MgB₂ is 54, and themagnetic penetration depth is 140-180 nm. One well known method ofmaking the superconductor of MgB₂ is to form a bulk of MgB₂ bycompression molding of powder of mono-crystal MgB₂, and then annealingthe bulk at a high temperature. Another method of forming thesuperconductor of MgB₂is to form a thin film of a MgB₂ compound by usingPLD (pulse laser deposition), and then annealing the thin film of MgB₂in the temperature range of 600° C.-1200° C. to make the filmsuperconductive. The PLD method focuses a pulsed laser beam on a borontarget to evaporate boron atoms and deposit the boron atoms on asubstrate. The boron atoms on the substrate then react with magnesiumatoms in high temperature magnesium vapor to form a MgB₂ film on thesubstrate. Further, the film is annealed at a high temperature, so as tobecome a superconductor. Another method of forming the superconductorfilm of a MgB₂ compound by using PLD is that which deposits magnesiumand boron atoms on a substrate by laser deposition, and sets thesubstrate in magnesium vapor. The magnesium and boron on the substratereact in the magnesium vapor to form a thin film of the MgB₂ compound onthe substrate. Annealing the MgB₂ film at a high temperature enables thefilm to have the features of a superconductor.

All prior art regarding fabrication of the superconductor of the MgB₂compound requires high temperature annealing, in the range of 600°C.-1200° C. The necessity of the annealing process of the MgB₂ film toproduce features of a superconductor makes fabrication of a device suchas a thin film integrated circuit on a substrate difficult. Further, ithas not been known that an as-grown film of MgB₂ on a substrate which ismade by simultaneous magnesium and boron sputtering, using a sputteringapparatus, has characteristics of a superconductor without annealing theas-grown film.

SUMMARY OF THE INVENTION

The present invention features a method of forming a superconductor filmof MgB₂ without annealing the as-grown film.

The present invention provides a method of forming an as-grown film of asuperconductor composed of the MgB₂ compound made with magnesium andboron ejected from a magnesium target and a boron target, respectively,in a simultaneous sputtering process. The as-grown film composed of thecompound of magnesium and boron can be a superconductor without beingannealed. The as-grown film can be composed of a MgB₂ compound, or thefilm can be composed of one or more compounds of magnesium and borondifferent from the MgB₂ compound, or a magnesium element and a boronelement in addition to the MgB₂ compound.

As mentioned above, a superconductor film of MgB₂ made using the presentinvention has features of a superconductor without being annealed, andits critical temperature is in the range of about 6K-29K. The presentinvention can be applied to fabricate an integrated circuit of asuperconductor film, because the high temperature annealing process isnot needed to make the as-grown film of the MgB₂ compound havingfeatures of a superconductor.

The objects, advantages and features of the present invention will bemore clearly understood by referencing the following detailed disclosureand the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sputtering apparatus to implement the first embodiment ofthe present invention.

FIG. 2 shows an example 1 of resistance-temperature characteristics of asuperconductor made by the present invention.

FIG. 3A shows temperature dependence of resistance-temperaturecharacteristics of a superconductor made by the present invention, wherethe substrate temperature is 320° C.

FIG. 3B shows temperature dependence of resistance-temperaturecharacteristics of a superconductor made by the present invention, wherethe substrate temperature is 350° C.

FIG. 3C shows temperature dependence of resistance-temperaturecharacteristics of a superconductor made by the present invention, wherethe substrate temperature is 380° C.

FIG. 4 shows temperature dependence of critical temperature and residualspecific resistance of a superconductor made by the present invention.

FIG. 5A shows resistance-temperature characteristics of a superconductorof example 2 made by the present invention using the sputteringapparatus shown in FIG. 1.

FIG. 5B shows resistance-temperature characteristics of a superconductorof example 2 made by the present invention using the sputteringapparatus shown in FIG. 1, where the axis of abscissas in FIG. 5B isenlarged and corresponds to the region of 0-50 K on the axis ofabscissas in FIG. 5A.

FIG. 6 shows a static-type sputtering apparatus for implementing thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a plan view of an apparatus to implement the presentinvention, that is a Carrousel-type sputtering apparatus forimplementing the present invention. As shown in FIG. 1, the apparatus iscomposed of a reaction room 1, pipes 5 and 6 for supplying argon gas, anion beam source 7, a buffering room 8, electric heaters 9 which are lampheaters for heating substrates 2, vacuum pumps 10 and 11, and rotarypumps 12 and 13.

Substrates 2, on which the superconductor is formed, are set verticallyto the plan view, on a holder. The holder is a hexagonal cylinder of sixsurfaces on which the substrates 2 are arranged. The cylinder rotatesaround an axis. The substrates 2 are, for example, Al₂O₃, MgO, SiO₂,etc. A magnesium target 3 of purity of 99.9%, a boron target 4 of purityof 99.5%, an aluminum target 15, and a niobium target 16′ are setvertically to the plan view on the wall of the reaction room 1. Eachtarget is faced to one of the substrates 2. The aluminum target 15 andthe niobium target 16′ are not used for forming a MgB₂ film. The ionbeam source 7 generates argon ions and the ions are irradiated to thesubstrates 2 to clean their surfaces.

The reaction room 1 is exhausted to vacuum by the vacuum pumps 10 and 11and the rotary pumps 12 and 13. Further, argon gas fills in the reactionroom 1. High voltage is applied between the magnesium target 3 and thesubstrate 2, and between the boron target 4 and the substrate 2. DCvoltage is applied between the magnesium target 3 and the substrate 2 sothat the magnesium target 3 is positive voltage. AC voltage is appliedbetween the boron target 4 and the substrate 2. The argon gas isdischarged between the target and the substrate of each target andsubstrate pair. The gas discharges can be run simultaneously at eachtarget and substrate pair, and each of the discharges can be controlledindependently. During the discharge, the substrates 2 are heated by theelectric heaters 9.

The discharge between the magnesium target 3 and the substrate 2 is DCdischarge, applying positive DC voltage to the magnesium target 3. Thedischarge between the boron target 4 and the substrate 2 is highfrequency discharge, thus applying high frequency alternating voltagebetween the boron target 4 and the substrate 2. The argon gas isdischarged between the magnesium target 3 and the substrate 2, andbetween the boron target 4 and the substrate 2. The argon ions generatedduring the discharge result in ion bombardments to the magnesium target3 and the boron target 4. As a result of the bombardments, atoms or ionsof magnesium and atoms or ions of boron are ejected from each target,respectively, when the argon atoms or ions impact the targets. Themagnesium atoms or ions and the boron atoms or ions react with eachother in the reaction room 1 to produce compounds of magnesium andboron. The compound of magnesium and boron is then deposited on thesubstrates 2, which rotate in a plane vertical to the plan view aroundan axis with high speed. Alternatively, the magnesium atoms or ions andthe boron atoms or ions are deposited on the substrates 2, and theyreact on the substrates 2 to produce the compound of magnesium andboron. In the explanation that follows, the sputtering process insimultaneous ion bombardments to the magnesium target and the borontarget is called simultaneous magnesium and boron sputtering.

The as-grown film formed by the above mentioned method is usuallypolycrystalline, which is composed of only MgB₂, or composed of the MgB₂compound and one or more than one of a magnesium element, a boronelement, or compounds of magnesium and boron different from MgB₂. Thefilm made by the present invention is usually composed of the componentsmentioned above. However, for the purpose of clarity, the presentinvention will be explained by a film composed of only a MgB₂ compound.

The substrates which are used in this embodiment of the presentinvention are Al₂O₃, MgO, or SiO₂. The temperature of each substrate isin the range of 250° C.-400° C. in the as-grown film forming process.The purity of argon gas is 99.9999%. The reaction pressure is in therange of 2-5 mTorr. The rotation speed of the substrates is 50 rpm. Theinput power for the discharge between the boron target and the substrateis high frequency AC power of 800 W. The input power for the dischargebetween the magnesium target and the substrate is DC power of 300 W. Thereaction time to form the film is 10-60 minutes.

FIG. 2 shows resistance-temperature characteristics of the MgB₂ filmmade by the simultaneous magnesium and boron spattering using theCarrousel-type sputtering apparatus shown in FIG. 1. The substrate ofthe experiment is Al₂O₃ (1102). The film forming conditions are thesubstrate temperature being 380° C., the input power for discharging thegas between the boron target and the substrate being AC power of 800W,the input power of the discharge between the magnesium target and thesubstrate being DC power of 300W, and the reaction pressure being 5mTorr (argon gas pressure). Under the above mentioned conditions, anas-grown film of MgB₂ is formed on the substrate, and the film hasfeatures of a superconductor without being annealed. Theresistance-temperature characteristics are shown in FIG. 2. The criticaltemperature Tc is about 28 K, which shows that the film issuperconductive at a high temperature.

FIGS. 3A, 3B, and 3C show the temperature dependence of theresistance-temperature characteristics. The resistance-temperaturecharacteristics are measured at various temperatures as a parameter.FIGS. 3A, 3B, and 3C show the resistance-temperature characteristics atthe substrate temperatures of 320° C., 350° C., and 380° C.,respectively, without annealing the as-grown film. The films are formedduring the simultaneous magnesium and boron sputtering by theCarrousel-type sputtering apparatus.

The substrate (1102) is Al₂O₃. The input power for the discharge betweenthe boron target and the substrate is AC power of 800W, and the inputpower for the discharge between the magnesium target and the substrateis DC power of 300W. The reaction pressure is 2 mTorr (in argon gas).The as-grown films formed by the simultaneous magnesium and boronsputtering in the apparatus shown in FIG. 1 have features of asuperconductor without being annealed. The RRR (Residual ResistanceRatio) is the resistance ratio at 300K and 40K, and shows the degree offilm fineness as a superconductor. It is known that the greater the RRRof the film is, the less the specific resistance of the film is, and thenearer to metal the film is.

FIG. 3A shows a case in which the substrate temperature is 320° C. Thefigure shows that the MgB₂ film of critical temperature Tc of 9K andRRR=0.99 can be formed without being annealed. FIG. 3B shows a filmformed on a substrate with a temperature of 350. This figure shows thatan as-grown film of MgB₂ of critical temperature Tc 19K and RRR 1.10 canbe formed without being annealed. FIG. 3C shows an as-grown film formedon a substrate that has a temperature of 380° C. This figure shows thatthe MgB₂ film of critical temperature Tc of 25.6K and RRR=1.14 can beformed without being annealed.

FIG. 4 shows the dependence of critical temperature and residualresistance ratio of the film formed by the present invention onsubstrate temperature. Relations of the critical temperature and theresidual resistance ratio to the substrate temperature are measured atvarious substrate temperatures to form the as-grown film of MgB₂. FIG. 4shows the results. The condition to form the film is the same as theconditions set forth for FIGS. 3A, 3B and 3C. In FIG. 4, the circledblack dots show the relationship between substrate temperature andcritical temperature, the uncircled black dots show the relationshipbetween residual resistance ratio and the substrate temperature. Thecritical temperature and the residual resistance ratio corresponding tothe same substrate temperature are those of the MgB₂ film on thesubstrate. The film formed on a substrate that has a temperature lowerthan 300° C. shows features of metal and does not show features of asuperconductor. In FIG. 4, the values of residual resistance ratio at300° C. are higher than the values at another substrate temperature.This is believed to occur because of the high magnesium rate near thesubstrate. For films formed on a substrate that has a temperaturebetween 320° C.-380° C. the high temperature superconductor feature ofresidual resistance ratio is 1.0-1.7.

As explained above, the as-grown film of MgB₂ formed on the rotatingsubstrate facing to the magnesium target and the boron target in thesimultaneous sputtering process, using Carrousel-type sputteringapparatus, has features of a high critical temperature superconductorwithout annealing the as-grown film. In the above mentioned embodimentof the present invention, the substrates are rotated with high speed,however, it has been found by the inventors of the present inventionthat an as-grown film of MgB₂ formed by the simultaneous sputteringprocess on a substrate kept static has features of a superconductor at ahigh temperature without annealing the as-grown film, similar to thoseof films formed on the rotating substrate with high speed in thesimultaneous sputtering process.

FIGS. 5A and 5B show resistance-temperature characteristics of MgB₂ filmwhich is made by a second embodiment of the present invention method.The axis of abscissas in FIG. 5B is enlarged and corresponds to theregion of 0-50 on the axis of abscissas in FIG. 5A. The as-grown film ofMgB₂ made by method of the second embodiment of the present invention isformed on an immovable substrate, which is different from the method ofembodiment 1 which uses rotating substrates. The resistance-temperaturecharacteristics in FIGS. 5A and 5B are those of MgB₂ film formed on animmobile substrate 2, using the Carrousel-type simultaneous sputteringapparatus shown in FIG. 1.

A substrate 2 is located so as to face to the middle between themagnesium target 3 and boron target 4 in the Carrousel-type sputteringapparatus, and is fixed at the position. The substrate on which the MgB₂film is formed is Al₂O₃ (1102). The conditions for forming an as-grownfilm of MgB₂ are a substrate temperature of 340° C. in the film formingprocess, the application of AC power of 800 W for the discharge betweenthe magnesium target and the substrate, the application of DC power of300 W for the discharge between the magnesium and the substrate, thereaction pressure of 5.0 mTorr in argon gas, and a reaction time of 20minutes for forming the film. As shown in FIGS. 5A and 5B, the as-grownfilm of MgB₂formed by the second embodiment of the present invention hasfeatures of the superconductor without annealing the as-grown film.

FIGS. 5A and 5B show that a film fabricated on the immobile substrate bysimultaneous magnesium and boron sputtering has features of asuperconductor. The film is fabricated by using Carrousel-typesimultaneous sputtering apparatus in a mode in which its substrateholder is not rotated. However, the MgB₂ film of the present inventioncan be formed with a static-type simultaneous sputtering apparatus.

FIG. 6 shows an outline of the static-type simultaneous sputteringapparatus. As shown in FIG. 6, the static-type spattering apparatus iscomposed of a reaction room 1, pipes 5 and 6 for supplying argon gas,ion beam source 7, buffering room 8, electric heaters 9 which are lampheaters for heating substrates 2, vacuum pumps 10 and 11, rotary pumps12 and 13, a substrate transportation rod 14, an axis of substrateholder 15′ which rotates the substrate 2 in a horizontal plane, ashutter 16, 17 which controls a film thickness, blocking the flow ofejected magnesium or boron atoms or ions, and a heater 17′ which heatsthe substrate 2.

The operation of the apparatus shown in FIG. 6 is explained as follows.Argon gas enters the reaction room 1. Positive DC voltage is appliedbetween the substrate 2 and the magnesium target 3. High frequency ACvoltage is applied between the boron target 4 and the substrate 2. Argongas is then discharged between the magnesium target 3 and the substrate2 and between the boron target 4 and the substrate 2, as a result of theapplied voltage. The discharge between the substrate 2 and the magnesiumtarget 3, and the discharge between the substrate 2 and the boron target4 can be controlled independently in the simultaneous magnesium andboron sputtering process. While the simultaneous magnesium and boronsputtering occurs, the argon ions bombard the magnesium target 3 and theboron target 4 to eject magnesium atoms and boron atoms, respectively,from each target. The atoms or ions of the magnesium and boron react toeach other in the reaction room 1 to produce molecules of a MgB₂compound. The MgB₂ molecules and an as-grown film of MgB₂ are thendeposited on the substrate. Alternatively, the atoms or ions of themagnesium or the boron ejected from each target, respectively, aredeposited on the substrate and the magnesium element and the boronelement and react to produce an as-grown film of MgB₂. The as-grown filmhas features of a superconductor without being annealed.

MgB₂ film formed by the method explained above is usuallypolycrystalline. However, a mono-crystalline or amorphous film of MgB₂which is formed by the simultaneous magnesium and boron sputteringprocess has features of a superconductor without being annealed. Suchfilms are involved in the scope of the present invention.

The many features and advantages of the present invention are apparentfrom the detailed specification and, thus, it is intended by theappended claims to cover all such features and advantages of theinvention which fall within the true spirit and scope of the invention.Further, since numerous modifications and changes will readily occur tothose skilled in the art, it is not desired to limit the invention tothe exact construction and operation illustrated and described, andaccordingly all suitable modification and equivalents falling within thescope of the invention may be included in the present invention.

What is claimed is:
 1. A method of forming a superconductor film,comprising: filling gas in a sealed container having therein a magnesiumtarget, a boron target and one or more substrates facing the targets;applying voltage between the magnesium target and the substrate todischarge the gas and between the boron target and the substrate todischarge the gas; and forming a film of a MgB₂ compound on thesubstrates, the MgB₂ compound generated from reaction of magnesiumejected in a sputtering process at the magnesium target and boronejected in a sputtering process at the boron target, wherein the film isformed on the substrate with a temperature of 300° C.-390° C., and thefilm of the MgB₂ compound has characteristics of a superconductoras-grown film without being annealed once the film has been formed. 2.The method of forming a superconductor film as in claim 1, wherein thefilm contains a compound of magnesium and boron stoichiometricallydifferent from the MgB₂ compound, a magnesium element, or a boronelement in addition to the MgB₂ compound.
 3. The method of forming asuperconductor film as in claim 1, wherein the film is polycrystalline,mono-crystalline, or amorphous.
 4. The method of forming asuperconductor film as in claim 1, wherein DC voltage is applied betweenthe magnesium target and the substrate to discharge the gas, and ACvoltage is applied between the boron target and the substrate todischarge the gas.
 5. The method of forming a superconductor film as inclaim 1, wherein a critical temperature of the film is 19K-29K.
 6. Themethod of forming a superconductor film as in claim 1, wherein thesubstrate is static in the film forming.
 7. The method of forming asuperconductor film as in claim 2, wherein the film is polycrystalline,mono-crystalline, or amorphous.
 8. The method of forming asuperconductor film as in claim 2, wherein DC voltage is applied betweenthe magnesium target and the substrate to discharge the gas, and ACvoltage is applied between the boron target and the substrate todischarge the gas.
 9. The method of forming a superconductor film as inclaim 2, wherein a critical temperature of the film is 19K-29K.
 10. Themethod of forming a superconductor film as in claim 2, wherein thesubstrate is static in the film forming.
 11. The method of forming asuperconductor film as in claim 1, wherein the magnesium target and theboron target are set separately in the sealed container.
 12. The methodof forming a superconductor film as in claim 1, wherein the container isa first polygonal cylinder and a second inner polygonal cylinder is setcoaxial with the center axis of the first polygonal cylinder, themagnesium target and the boron target are placed on inside walls of thefirst polygonal cylinder, respectively, and substrates are placed onoutside surfaces of the second inner polygonal cylinder.